This invention relates generally to implantable medical devices and more particularly, it relates to an implantable cardiac defibrillator employing a switched capacitor filter stage which includes a switch structure so as to produce a low charge-injection induced offset voltage on the output of an operational amplifier. In particular, the present invention is directed to an implantable cardiac defibrillator device which utilizes the present switched capacitor filter stage for processing heart signals from the atrium and/or ventricle.
In recent years, there has been substantial progress made in the research and development of defibrillating devices for providing an effective medical response to various disorders, such as ventricular fibrillation. Research effort has also been made toward developing improved sensing techniques for reliably monitoring heart activity so as to determine when a defibrillating high energy shock is required. However, the implantable cardiac defibrillators of the prior art used comparatively simple sensing circuits. These prior art sensing circuits would typically include switched capacitor circuits.
In general, switched capacitor circuits combine switches implemented with a pair of CMOS transistors and capacitors where the transistors are controlled by true and complementary clock pulse signals. When the switched capacitor circuits are used to construct a switched capacitor filter stage containing an operational amplifier there is suffered the disadvantage of creating a charge-injection induced offset voltage on the output of the operational amplifier when the switch transistors are turned off. This is caused by charge injection due to carriers released from the channel and to coupling through gate-to-diffusion overlap capacitances. For the purposes of completeness, reference is made to an article entitled "Charge Injection in Analog MOS Switches" by George Wegmann, Eric A. Vittoz and Fouad Rahali in IEEE Journal of Solid State Circuits, Vol. SC-22 No. 6, December, 1987, pp. 1091-1097, which discusses in detail the switch induced charge injection problems. It would therefore be desirable to provide a switched capacitor filter stage which reduces or eliminates the problem of charge-injection induced offset voltage associated with the output of an operational amplifier.
The present invention provides an implantable cardiac defibrillator employing a switched capacitor filter stage which includes a unique switch structure so as to produce a low charge-injection induced offset voltage on the output of an operational amplifier. This is achieved by inserting MOS charge steering resistors in series with the capacitors that are coupled to a sensitive node of the operational amplifier. As a result, the stored channel charges tend to flow to the low impedance node, that is directed away from the sensitive nodes, thereby reducing the effect of charge injection when the transition of the leading and trailing edges of the clock pulse signals that control the switching transistors are slowed down. For purposes of completeness, reference is made to an article entitled "High Resolution Switched Capacitor D/A Converter" by Roubik Gregorian in Microelectronics Journal, Vol. 12, No. 2, 1981, pp. 10-13, where the basic switched capacitor configuration, in non-filtering form, is presented.